The development of highly absorbent members for use as disposable diapers, adult incontinence pads and briefs, and catamenial products such as sanitary napkins, are the subject of substantial commercial interest. A highly desired characteristic for such products is thinness. For example, thinner diapers are less bulky to wear, fit better under clothing, and are less noticeable. They are also more compact in the package, making the diapers easier for the consumer to carry and store. Compactness in packaging also results in reduced distribution costs for the manufacturer and distributor, including less shelf space required in the store per diaper unit.
The ability to provide thinner absorbent articles such as diapers has been contingent on the ability to develop relatively thin absorbent cores or structures that can acquire and store large quantities of discharged body fluids, in particular urine. In this regard, the use of certain absorbent polymers often referred to as "hydrogels," "superabsorbents" or "hydrocolloid" material has been particularly important. See, for example, U.S. Pat. No. 3,669,103 (Harper et al), issued Jun. 13, 1972, and U.S. Pat. No. 3,670,731 (Harmon), issued Jun. 20, 1972, that disclose the use of such absorbent polymers (hereafter "hydrogel-forming absorbent polymers") in absorbent articles. Indeed, the development of thinner diapers has been the direct consequence of thinner absorbent cores that take advantage of the ability of these hydrogel-forming absorbent polymers to absorb large quantities of discharged aqueous body fluids, typically when used in combination with a fibrous matrix. See, for example, U.S. Pat. No. 4,673,402 (Weisman et al), issued Jun. 16, 1987 and U.S. Pat. No. 4,935,022 (Lash et al), issued Jun. 19, 1990, that disclose dual-layer core structures comprising a fibrous matrix and hydrogel-forming absorbent polymers useful in fashioning thin, compact, nonbulky diapers.
Prior to the use of these hydrogel-forming absorbent polymers, it was general practice to form absorbent structures, such as those suitable for use in infant diapers, entirely from wood pulp fluff. Given the relatively low amount of fluid absorbed by wood pulp fluff on a gram of fluid absorbed per gram of wood pulp fluff, it was necessary to employ relatively large quantities of wood pulp fluff, thus necessitating the use of relatively bulky, thick absorbent structures. The introduction of these hydrogel-forming absorbent polymers into such structures has allowed the use of less wood pulp fluff. These hydrogel-forming absorbent polymers are superior to fluff in their ability to absorb large volumes of aqueous body fluids, such as urine (i.e., at least about 15 g/g), thus making smaller, thinner absorbent structures feasible.
Prior absorbent structures have generally comprised relatively low amounts (e.g., less than about 50% by weight) of these hydrogel-forming absorbent polymers. See, for example, U.S. Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989 (preferably from about 9 to about 50% hydrogel-forming absorbent polymer in the fibrous matrix). There are several reasons for this. The hydrogel-forming absorbent polymers employed in prior absorbent structures have generally not had an absorption rate that would allow them to quickly absorb body fluids, especially in "gush" situations. This has necessitated the inclusion of fibers, typically wood pulp fibers, to serve as temporary reservoirs to hold the discharged fluids until absorbed by the hydrogel-forming absorbent polymer.
More importantly, many of the known hydrogel-forming absorbent polymers exhibited gel blocking. "Gel blocking" occurs when particles of the hydrogel-forming absorbent polymer are wetted and the particles swell so as to inhibit fluid transmission to other regions of the absorbent structure. Wetting of these other regions of the absorbent member therefore takes place via a very slow diffusion process. In practical terms, this means acquisition of fluids by the absorbent structure is much slower than the rate at which fluids are discharged, especially in gush situations. Leakage from the absorbent article can take place well before the particles of hydrogel-forming absorbent polymer in the absorbent member are fully saturated or before the fluid can diffuse or wick past the "blocking" particles into the rest of the absorbent member. Gel blocking can be a particularly acute problem if the particles of hydrogel-forming absorbent polymer do not have adequate gel strength and deform or spread under stress once the particles swell with absorbed fluid. See U.S. Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989.
This gel blocking phenomena has typically necessitated the use of a fibrous matrix in which are dispersed the particles of hydrogel-forming absorbent polymer. This fibrous matrix keeps the particles of hydrogel-forming absorbent polymer separated from one another. This fibrous matrix also provides a capillary structure that allows fluid to reach the hydrogel-forming absorbent polymer located in regions remote from the initial fluid discharge point. See U.S. Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989. However, dispersing the hydrogel-forming absorbent polymer in a fibrous matrix at relatively low concentrations in order to minimize or avoid gel blocking can lower the overall fluid storage capacity of thinner absorbent structures. Also, absorbent cores comprising hydrogel-forming absorbent polymers dispersed uniformly throughout the fibrous matrix will typically not have the ability to rapidly acquire and distribute fluids during "gush" situations or when the core has become saturated from prior discharges of body fluids.
The need for rapidly acquiring and distributing discharge body fluids has led to the development of dual-layer core structures noted above. These dual-layer core structures basically comprise: (1) an upper fibrous layer adjacent to the fluid pervious topsheet that is substantially free of hydrogel-forming absorbent polymers that acquires the discharged fluid; and (2) a lower layer that stores this acquired fluid and is typically either: (a) a fibrous matrix having hydrogel-forming absorbent polymers uniformly dispersed therein; or (b) a laminate structure where hydrogel-forming absorbent polymer is between two tissue layers. See, for example, U.S. Pat. No. 4,673,402 (Weisman et al), issued Jun. 16, 1987. See also U.S. Pat. No. 4,935,022 (Lash et al), issued Jun. 19, 1990 and U.S. Pat. No. 5,217,445 (Young et al), issued Jun. 8, 1993, where certain chemically stiffened curly, twisted cellulosic fibers are used in this upper layer to provide improved acquisition and distribution performance. Another variation is to "profile" the absorbent core such that there is an acquisition zone substantially free of hydrogel-forming absorbent polymers in the fluid discharge area and a storage area having dispersed therein hydrogel-forming absorbent polymers that is in fluid communication with the acquisition zone. See U.S. Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989 and U.S. Pat. No. 5,047,023 (Berg), issued Sep. 10, 1991.
Even with the fluid handling improvements provided by these prior absorbent designs, it has been found that the ability to readily acquire fluid diminishes rapidly as the absorbent core becomes saturated with aqueous body fluids. This occurs because the void spaces between fibers and the hydrogel-forming absorbent polymers in the absorbent core become partially filled with fluid during the first "gush" and therefore can not rapidly accept the necessary volume of fluid during subsequent "gushes". Furthermore, the risk of leakage increases as the number of loads increases.
Another problem that can occur with some prior absorbent core designs is a phenomenon referred to as "rewet." Rewet occurs when there is acquired fluid that is freely mobile and available in that portion of the absorbent core adjacent the topsheet. This is typically experienced as the absorbent core becomes saturated with acquired fluid. Under mechanical pressure from the wearer of the article, this mobile fluid is pumped out of the absorbent core and upwards through the topsheet. As a result, the topsheet becomes "rewetted" with this pumped fluid such that there is not adequate topsheet dryness.
Accordingly, it would also be desirable to provide an absorbent core that: (1) has an absorbent material, capable of swelling upon absorbing discharged body fluid to form a fluid acquisition zone, in the absorbent core for desired total fluid capacity and thinness; (2) is able to acquire discharged fluid rapidly during "gush" situations, even when the core has become saturated in the loading area from prior discharges of fluids; and (3) when incorporated into an absorbent article, preferably minimizes rewetting of the topsheet.